There could be several things that could be causing it. It may be a software related or hardware, I would try a different SD card if you have access to one to make sure it is not just the Make and Model of the new 16GB SD card you are using, If it has been replaced by warranty 3 times it may be incompatible with the Play. Also when you switched SD cards did you copy everything from the 8GB to the 16GB? And when it stops reading are you able to mount it again or do you have to remove it from the phone and replace it to get it working again?